1. Field of the Invention
The present invention relates to a rotating angle detecting apparatus detecting a rotating angle of a detected subject by using an angular resolver, and an electric power steering apparatus assisting a steering by attaching the angular resolver to a torsion bar connected to a steering wheel, detecting a steering torque on the basis of a rotating angle determined by the angular resolver, and determining an assist amount so as to control a motor.
2. Description of the Related Art
In conventional, there has been known an electric power steering apparatus which reduces a steering force by a steering wheel by applying an assist force generated by a motor to a steering mechanism connected to a steering shaft. In the electric power steering apparatus mentioned above, there is a case that a torque sensor determining a steering torque on the basis of a torsion of a torsion bar is employed. As a rotating angle sensor of this kind of torque sensor, there has been generally employed an angular resolver having a high mechanical reliability. As the electric power steering apparatus in accordance with the structure mentioned above, there is JP HEI 11-321689 A.
Since an output from the angular resolver is constituted by alternating-current analogue signals of a sin phase or a cos phase, the output is generally raised by applying an offset voltage to the sin phase and the cos phase in accordance with an offset voltage applying circuit so as to convert into a direct-current analogue signal and thereafter is converted into a digital signal via an A/D converter, for the purpose of allowing a process by a CPU. JP 2000-132226 A shows a structure for detecting a disconnection of the resolver. JP HEI 11-37797 A shows a structure for detecting a disconnection of the sensor and a ground fault.
3. Prior Art    JP HEI 11-321689 A    JP 2000-132226 A    JP HEI 11-37797 A
In the electric power steering apparatus, in order to increase a reliability, it is desirable to detect the disconnection and the ground fault between the angular resolver detecting the steering torque and the CPU and employ a countermeasure of gradually lowering the assist force or the like when the disconnection or the ground fault is occurred. For example, when a harness comes off due to a vibration or the like and the disconnection is generated, or when a wire is in contact with a vehicle body in the case that the vehicle runs on a curb stone and the ground fault is generated, it is necessary to stop the assist of the steering on the basis of a signal from the angular resolver.
In a method of detecting the disconnection and the ground fault, since the signal from the angular resolver rised by applying the offset voltage by the offset voltage applying circuit changes at a time of the disconnection and the ground fault with respect to a normal time, it is possible to detect on the basis of this change. Since a direct-current component resistance value of an output coil structuring the angular resolver becomes infinitely large and a potential change is large at a time of the disconnection, it is possible to accurately detect. However, even if it is intended to detect the ground fault on the basis of the voltage change, the direct-current component resistance value of the output coil structuring the angular resolver is small and the change amount of the potential is small even when the direct-current component resistance value becomes zero, so that the change amount of the voltage is not distinguished from a variation of the signal potential at a normal time on the basis of the angular resolver itself, a variation of a characteristic of an electronic circuit and a temperature change. Taking into consideration a risk of a steering assist stop caused by an erroneous fail detection, the ground fault can not be actually detected.